The 'absheight' flag was added years ago for the floatlands of 'indev'mapgen (now deleted). The feature mirrored all ore placement around y = 0to place ores in floatlands.

In MTG we now use dedicated ore registrations for floatlands.

The feature is crude, inflexible, problematic and very rarely used, italso makes ore vertical range code more complex.Minetest 0.5 is a good chance to remove the feature.

I remember talking with hmmmm about this, we agreed it was a crude and not much usable feature, i suggested to remove it but hmmmm was concerned about breakage, 0.5 is a good chance.

Generally ores are meant to be below y = 0, with this flag enabled any ores between y = 0 and y = -256 appear in hills and mountains between y = 0 and y = 256, the only way to avoid this is to keep ores below y = -256, which is bad for gameplay. The feature is therefore almost useless.

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The flag is now simply non-functional, if it is in your ore registrations nothing will break, the mirrored ores will just not appear.